Rollup merge of rust-lang#32164 - nikomatsakis:fewer-errors, r=eddyb

Do not report errors from regionck if other errors were already reported

Do not report errors from regionck if other errors were already reported during the lifetime of this inferencer. Fixes rust-lang#30580.

r? @arielb1

src/librustc/middle/infer/mod.rs

@@ -1120,11 +1120,15 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                                         .map(|method| resolve_ty(method.ty)))
     }
 
+    pub fn errors_since_creation(&self) -> bool {
+        self.tcx.sess.err_count() - self.err_count_on_creation != 0
+    }
+
     pub fn node_type(&self, id: ast::NodeId) -> Ty<'tcx> {
         match self.tables.borrow().node_types.get(&id) {
             Some(&t) => t,
             // FIXME
-            None if self.tcx.sess.err_count() - self.err_count_on_creation != 0 =>
+            None if self.errors_since_creation() =>
                 self.tcx.types.err,
             None => {
                 self.tcx.sess.bug(
@@ -1147,7 +1151,14 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                                              free_regions: &FreeRegionMap,
                                              subject_node_id: ast::NodeId) {
         let errors = self.region_vars.resolve_regions(free_regions, subject_node_id);
-        self.report_region_errors(&errors); // see error_reporting.rs
+        if !self.errors_since_creation() {
+            // As a heuristic, just skip reporting region errors
+            // altogether if other errors have been reported while
+            // this infcx was in use.  This is totally hokey but
+            // otherwise we have a hard time separating legit region
+            // errors from silly ones.
+            self.report_region_errors(&errors); // see error_reporting.rs
+        }
     }
 
     pub fn ty_to_string(&self, t: Ty<'tcx>) -> String {

src/test/compile-fail/issue-30580.rs

@@ -0,0 +1,27 @@
+// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test that we do not see uninformative region-related errors
+// when we get some basic type-checking failure. See #30580.
+
+pub struct Foo { a: u32 }
+pub struct Pass<'a, 'tcx: 'a>(&'a mut &'a (), &'a &'tcx ());
+
+impl<'a, 'tcx> Pass<'a, 'tcx>
+{
+    pub fn tcx(&self) -> &'a &'tcx () { self.1 }
+    fn lol(&mut self, b: &Foo)
+    {
+        b.c; //~ ERROR no field with that name was found
+        self.tcx();
+    }
+}
+
+fn main() {}